ELSEWER Carbohydrate Research 258 (1994) 293-297 Note Stereoselective synthesis of p-D-glycopyranosyl+serinate or -threoninate derivatives with an unusual migration Lajos Szab6, Robin Polt * Chenai.wy Department, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ Universi~ of Arizona, Tucson, Arizona 85721 USA (Received April 29th 1993; accepted in revised form December 19th, 1993) Stereoselective glycosylation is the most important aspect of complex carbohy- drate synthesis [l-4]. Generally, l-O-a@ glycopyranoses have been used only as starting materials or intermediates for glycosylation reactions. In recent years, however, there has been increased interest in acyl-glycose [5] derivatives with biological activity, i.e., glycosylated opioid peptides [6,7] and derivatives of glu- curonic acids [8-111. The synthesis of O-linked glycopeptides [2,4,12] is complicated by the acid lability of the glycosidic linkage, and the base lability of the @hydroxy amino acid residue. Additional complications arise due to the poor nucleophilicity of the usual N-acylated (e.g., Boc, Cbz, Fmoc protection) serine and threonine substrates. Because of this decreased reactivity, harsh reaction conditions are required to effect glycosidic bond formation, and the yields suffer, as well as the (Y- vs. /3-glycoside selectivity [13]. In order to avoid these problems, we have used the benzophenone Schiff base esters 1141of serine and threonine which have proved to be more reactive in both 1,2&s and frans-glycosylations 115,161. Various metal halides (Lewis acids) have been used as glycosylation promoters [17], but the use of ZnCl, has only been applied to very special cases - the preparation of aryl-1-thioglycosides [18], glycosides [19], and 2-amino-Zdeoxy-glu- copyranosides [20]. In a classic study, Lemieux studied the anomerization of glycosides in the presence of a variety of Lewis acids, including Z&l, [21]. Our studies show that ZnCl, can be an efficient promoter in the synthesis of serine or threonine 1,2-traras-0-a&derivatives of carbohydrates via migration. In a typical procedure, a solution of acetobromo sugar (l-4, 1.5 mmol), fused and powdered ZnCl, (1.5 rnmol), and the glycosyl acceptor (5 and 6, 1 mmol) in CH,CI, (10 mL) were stirred at room temperature under rigorously anhydrous * Corresponding author. 0008-6215/94/$07.00 Q 1994 Elsevier Science B.V. AU rights resewed SSDI 0008-6215(94)00006-2